18 4Sum

class Solution:
# @param {integer[]} nums
# @param {integer} target
# @return {integer[][]}
def fourSum(self, nums, target):
"if we adopt left-><-right method, the time complexity is O(n**3) which is not so perfect for this problem written in python(http://www.cnblogs.com/zuoyuan/p/3699384.html), thus, we can sacrifice the space for time,O(n**2)"

dic,collection={},set()
l=len(nums)
nums.sort()
if l<4:
return []
for i in range(l-1):
for j in range(i+1,l):
if nums[i]+nums[j] not in dic:
dic[nums[i]+nums[j]]=[(i,j)]
else:
dic[nums[i]+nums[j]].append((i,j))

for k in range(l-1):
for m in range(k+1,l):
tmp=target-nums[k]-nums[m]
if tmp in dic:
for n in dic[tmp]:
if n[0]>m:
collection.add((nums[k],nums[m],nums[n[0]],nums[n[1]]))
return [list(p) for p in collection]